Al—Li alloys and certain other highly alloyed aluminum alloys have traditionally been melted using induction melt-furnace technology, generally coreless or channel induction types. Due to the chemical activity of lithium in aluminum, standard furnaces, the designs-of-combustion gas fired furnaces, are not used. To melt the Al—Li alloys, indirect inductively generated heat is applied using an induction furnace's electromagnetic field, where the metal in the furnace couples with the magnetic field to generate heat. Coreless induction furnaces typically have a continuous coil, usually copper, surrounding the circumference of the body of the furnace. A channel induction furnace has the induction coil mounted externally to the main body of the furnace, and uses a pass-through method to transfer molten metal through a heating zone. Channel induction furnaces are generally larger than coreless induction furnaces, and were developed because the coreless induction furnaces have a practical size limitation. For both above types of induction furnaces, the heat energy developed via the magnetic field as well as from the molten metal itself requires the induction coils to be liquid cooled using water or glycol or mixtures thereof. Water is generally used as the coolant but this creates a safety issue if a furnace lining failure occurs. The molten metal could penetrate the furnace lining and reach the cooling coil, and if the molten metal penetrates the cooling coil itself, an aluminum explosion could result from aluminum to water contact. A number of publications, including the Guidelines for Handling Molten Aluminum published by The Aluminum Association (USA), discuss explosions and the requirement to keep molten aluminum away from water. When melting and processing Al—Li alloys, the potential for catastrophic explosions with water are greater than for conventional (non-lithium containing) aluminum alloys. For this reason, several furnace manufacturers offer furnace cooling systems that use coolant other than water for coil cooling, particularly halogenated glycols.
Typical aluminum alloys use standard industrial refractories as working lining for the induction furnace. These include mullite, alumina and silica-based materials installed as cast-in-place linings, refractory brick and mortar linings, and precast made-to-fit crucibles. These materials are inserted into the furnace body, along with other intermediate materials to separate the molten aluminum from contacting the furnace induction coils. The lining material exposed to the molten metal is considered an expendable, and is periodically replaced as needed. The backup materials (which do not normally come in direct contact with molten aluminum alloy) have generally longer life than those that come in direct contact, and the backup materials are generally not routinely replaced during a working lining replacement.
Al—Li alloys require special working lining refractories due to the chemical activity of lithium in aluminum. Magnesium oxide (MgO) and alumina (Al2O3) based refractories are typically used for coreless induction furnaces, while silicon carbide (SiC) based refractories are used in the non-magnetic region away from inductor. For small, laboratory sized induction furnaces, SiC crucibles are used. The primary drawback with MgO is its relatively low heat fatigue resistance. This necessitates that the furnace be maintained hot and not drained on regular basis. This also poses a problem during alloy change as the furnace generally cannot be cooled without cracking the refractory after it has been used for melting metal. As a rule of thumb, if the MgO furnace lining is allowed to cool below 1000° F. it will crack and become unusable. Because Aluminum melts at 1260° F. and is alloyed at 1400° F., the lining has to be permanently kept at 1400° F. Thus, extraneous means are necessary to maintain heat in the furnace at all times, even when not in use, as well as between furnace operating cycles.
Furnaces incorporating technologies other than induction have been employed for melting Al—Li alloys, including resistance heated vacuum furnaces. Aluminum-lithium alloying processes have also used techniques of post-furnace-in-line alloying of the lithium such that the lithium does not contact or contaminate traditional furnace refractories; see U.S. Pat. No. 4,248,630. Refractory products containing free silica and/or phosphates are especially bad when used in conjunction with Al—Li melts, as the lithium preferentially attacks these materials, which leads to almost immediate destruction of the ceramic.
U.S. Pat. No. 5,028,570 (“the '570 patent”) teaches that aluminum-lithium alloys that are used in aerospace applications typically contain about 2-3 percent lithium, which significantly increases the strength of aluminum and decreases the weight of the alloy relative to pure aluminum. Only two refractories have been found that can provide a reasonable containment of these alloys. These are oxide-bonded magnesia and silicon-nitride bonded silicon carbide. The '570 patent describes silicon nitride bonded MgO, which is more corrosion resistant to molten Al—Li. Additionally dry vibratory mixes consisting of silicon carbide and alumina (manufactured and marketed by Allied Mineral Corporation, Columbus, Ohio and Saint Gobain Corp of America, Amherst, Mass.) are also employed in conventional coreless induction furnaces used for melting aluminum lithium alloys. Pre-cast and fired crucibles made out of tabular alumina (containing 96 percent of high purity tabular alumina, approximately 2 percent silica and 2 percent titanium oxide) are also in use as main lining material of a melt-containing vessel in aluminum lithium applications. However, all of the above mentioned refractories react with aluminum lithium alloys and produce alloys that tend to and develop spalling coupled with a network of hairline cracks. The problem arises when during charging or skimming or furnace wall cleaning—the refractory undergoes further mechanical abuse. The mechanical abuse enhances the hairline cracks present in the refractory from thermal cycling. This, coupled with chemical reaction between the refractory lining and the lithium containing melt, and further the furnace filled with the low melting eutectics from the melt, gives rise to thicker sections of the entrapped semi-solid, solid, semi-liquid or fully liquid fins of the alloy to form a network within the refractory lining of the vessel wherein such network is slowly progressing to the outer wall of the lining. Because the inductive energy can readily couple with the network of aluminum or aluminum alloy fins of certain thickness (over 1.5 mm) trapped inside the lining, when the furnace is operated at a particular frequency and at required input of electromagnetic power, the network of fins becomes superheated and rapidly advances to the outer boundary of the refractory lining. The resulting failure of the refractory lining becomes a strong limiting factor in the life of the furnace. If the failure of the refractory lining were to present itself only as a pure expense, it would only remain as an addressable cost item. However, the sudden advancement of liquid aluminum lithium alloy towards the induction coil through the damaged refractory lining of the melt-containing vessel presents a catastrophic explosion possibility if it were to reach even one or two turns of the induction coil. Thus, absent a refractory material that is chemically inert to molten aluminum lithium alloys, there remains a distinct need to isolate the induction coil completely from the refractory lining of the melt-containing vessel.
Typical induction furnaces operate at very low electrical frequencies. To obtain stirring of the melt during the melting process, a low frequency is important to obtain a rapid melt rate. However, the rapid melt rate makes the task of keeping the lithium in the melt more difficult unless tightly controlled inert atmosphere is continuously maintained above the melt. U.S. Pat. No. 5,032,171 describes the use of low frequency induction power to stir the melt vigorously such that removal of lithium is promoted. When using a higher frequency induction furnace, less stirring occurs, as movement of the molten metal is an inverse function of operating frequency. Higher frequency results in less stirring, however, higher frequency also results in coupling more of the induction energy closer to the inside wall of the melt-containing vessel, and if the fins are present strong coupling and thereby superheating of the fins results which additionally accelerates the degradation of the refractory. Thus using lower frequency in the power source cannot mitigate the degradation of the refractory lining. Another issue related to using low frequency (to achieve rapid melting) is the resultant forceful stirring that leads to the entrainment of non-metallic particles and undesirable oxides in the melt. Because lower frequencies result in more melt stirring, an operating frequency compromise is often made to suit the operation but only at the expense of doing more damage to the refractory lining and weakening the control over the bath temperature.
For scrap melting, where quality is secondary to productivity, lower frequencies are typically used. When producing high quality melts, higher frequencies are used to reduce undesirable stirring, at the expense of productivity.
Another fundamental factor connected with Al—Li melts is the degree of hydrogen solubility in the molten Al—Li alloy. Because hydrogen is completely soluble in pure molten lithium (which melts at only 400° F.), the molten Al—Li alloy at 1400° F. captures significant amount of hydrogen in the alloyed melt. For example, a furnace melt of typical non lithium containing aerospace aluminum alloy AA 7050 will have hydrogen content in a freshly prepared melt in a reverberatory melting furnace of 0.5 cc/100 gms of molten alloy. As compared to this, the amount of dissolved hydrogen in a freshly prepared melt of 1.2 percent Li alloy melted inside a controlled atmosphere induction furnace is 1.5 cc/100 gms of molten alloy. Hydrogen in the regular aluminum alloys as well as in the aluminum lithium alloys is deleterious because it gives rise to porosity in the cast products. Such porosity in the cast condition of the alloy is difficult to heal during thermo-mechanical processing and affects the strength, ductility, corrosion resistance and fatigue resistance of the finished products made from such castings that carry higher amount of hydrogen. Besides the hydrogen coming in to the molten Al—Li alloy through the addition of lithium, there is another source that contributes to hydrogen pick up in the melt. This source is chemical in nature. Al—Li melts are extremely powerful reducing agents and they strip away bonded hydrogen from components of the refractory used in the melt-containing vessel. The binding agents used in the preparation of the melt vessel refractory typically contain caustic or phosphoric acids or water or organic activators, all of which contain some amount of bonded hydrogen. This hydrogen can be stripped away by Al and Li atoms and is readily absorbed by the melt with simultaneous formation of Al—Li oxides, carbides, borides, etc. A representative chemical reaction is 2Al+3H2O=Al2O3+6H, whereby large quantity of hydrogen is liberated and retained by the melt.
Besides the above two contributors of hydrogen, there is yet another source of hydrogen transport in to the melt. This transport happens through the refractory of the melt-containing vessel of any standard induction-melting furnace. The transport happens readily because a) there is higher partial pressure of hydrogen outside the outer wall of the refractory (which sits within the coil grout) than it is on the inside wall of the vessel refractory lining (which is in contact with the melt), b) hydrogen being the smallest atom, the kinetics and the coefficient of hydrogen transfer are very favorable to maintain a continuous diffusion of hydrogen, driven by the hydrogen partial pressure difference. The coil grout is always in direct contact with the plant atmosphere and depending on the humidity (which is always high in an aluminum cast house since water is used as a heat extraction media), a reasonable amount of moisture (relative humidity 20 percent or higher) and thereby hydrogen is reminiscent on the outside surface of the coil grout. To reduce such hydrogen pick-up in the melt transported through the refractory, the industry has found it necessary to employ another electrical holding furnace to degas the specialty alloy melts including Al—Li melts prior to casting. Such holding furnaces are of three designs, (i) vacuum is either applied on top of the Al—Li bath surface, or (ii) the exterior of melt-containing vessel is maintained in vacuum, or (iii) vacuum is applied at both locations, in the interior as well as the exterior.